JP2002371313A - Method for smelting molten stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for smelting molten stainless steel, which reduces Al concentration in the molten stainless steel, accelerates desulfurization, facilitates smelting the molten stainless steel into an extreme low Al and low S content, reduces Cr2 O3 concentration in slag, and enables resource recovery of the generated slag. SOLUTION: The method for smelting the molten stainless steel, which smelts a crude molten steel through primary decarbonization, by blowing oxygen into a molten pig iron and a ferrochrome alloy charged in a converter type vessel 10, taps the crude molten steel out into a ladle 21, refines the crude molten steel through the secondary decarbonization and deoxidation treatment with the use of a secondary refining furnace 30, is characterized by reducing the slag through adding a silicon-containing alloy to the slag, which is generated after the primary decarbonization refinement by oxygen-blowing, then adding Al so that Al concentration in the crude molten steel may become 0.05-0.2 wt.%, desulfurizing it, tapping it out into the ladle 21, and adding no Al during heating up and deoxidation treatment in sequential secondary decarbonization refinement with the use of the secondary refining furnace 30.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method for reducing Cr in slag produced by primary decarburization refining with a Si-containing alloy using a converter type refining vessel, adding Al, desulfurizing, In the secondary decarburization refining performed, by not adding Al,
TECHNICAL FIELD The present invention relates to a method for producing a molten stainless steel for producing a very low Al and low S molten stainless steel.

[0002]

2. Description of the Related Art Conventionally, a stainless steel sheet produced by casting a molten stainless steel containing chromium or nickel into molten iron by continuous casting or the like and subjecting the slab to rolling processing has a problem of corrosion resistance and glossiness. It is widely used for oil country tubular goods and building materials. Corrosion resistance of this stainless steel plate,
The characteristics of toughness and the like are as follows: when smelting stainless steel using a refining furnace, the aluminum (A
1) coarse oxide-based inclusions resulting from
Since sulfides are formed, there are problems that the corrosion resistance of the stainless steel plate is deteriorated and the toughness of the stainless steel plate is reduced. As a countermeasure against this, as described in JP-A-10-195605, carbon is added in an amount of 0.005 to 0.1%.
A stainless steel sheet containing 12 to 16% by weight of chromium (Cr), 0.005 to 0.2% by weight of Al, and 0.015% by weight of sulfur (S) has been proposed. A method of suppressing coarse inclusions, which are products of Al deoxidation, and preventing corrosion and corrosion cracking due to sulfides by using the method has been performed.

[0003]

However, the stainless steel sheet disclosed in Japanese Patent Application Laid-Open No. 10-195605 describes the composition of the stainless steel sheet.
When smelting stainless steel of that composition, during the refining process,
Ultra-low Al and low S concentration stainless steel molten steel cannot be easily melted using the same converter type refining furnace to reduce the Al concentration of molten stainless steel and promote desulfurization. There is a problem that the efficiency of reduction and recovery of chromium oxide (Cr ₂ O ₃ ) therein is low, and the generated slag cannot be used as a resource.

The present invention has been made in view of such circumstances, and aims to reduce the Al concentration in molten stainless steel and promote desulfurization to facilitate the production of extremely low Al and low S molten stainless steel.
It is an object of the present invention to provide a method for smelting stainless steel that reduces the concentration of Cr ₂ O _{3 in} slag and enables the generated slag to be recycled.

[0005]

According to the present invention, there is provided a method for producing molten stainless steel according to the present invention, which comprises charging a hot metal and a ferrochrome alloy into a converter type vessel and using an upper blowing lance for the hot metal and the ferrochrome alloy. After blowing the molten iron and performing melting and primary decarburization refining of the molten iron and the ferrochrome alloy to produce the crude molten steel, the crude molten steel is discharged into a ladle, and then the secondary molten metal is subjected to a secondary refining furnace. In the method for producing molten stainless steel in which secondary decarburization refining and deoxidation treatment are performed by using the above-mentioned slag, a silicon-containing alloy is added to slag generated after performing the primary decarburization refining with the blowing acid. After the reduction treatment, the Al concentration in the crude molten steel is 0.05 to 0.2% by weight.
Al is added so as to obtain a desulfurization treatment, and then the steel is output to the ladle, and Al is not added at the time of heating and deoxidizing treatment in the subsequent secondary decarburization refining using the secondary refining furnace. Perform processing. By this method, Cr ₂ O contained in the slag generated during the primary decarburization refining by the converter type vessel
₃ can be reduced with a reducing agent containing silicon, Cr can be recovered in the crude molten steel, and the Al concentration in the crude molten steel is reduced to 0.05 to 0.1.
Since desulfurization treatment is performed at 2% by weight, desulfurization can be promoted by lowering the oxygen potential. Further, during the heating period of the secondary decarburization refining, if the heating of Al is performed, A in the slag is increased.
l ₂ O ₃ concentration increased, and Al ₂ O
₃ is reduced by another deoxidizing agent such as a Si alloy to increase the Al concentration in the molten steel, so that it cannot be reduced to an extremely low Al. Therefore, by not adding Al in the heating step performed in the secondary decarburization refining by the secondary refining furnace and in the deoxidizing treatment performed after the secondary decarburization refining, the Al content in the final molten stainless steel is extremely low. Al and low S. If the Al concentration is less than 0.05% by weight, the oxygen potential in the slag increases, the desulfurization reaction deteriorates, and the ultimate S concentration increases. On the other hand, when the Al concentration is 0.2% by weight.
Is exceeded, the desulfurization effect becomes saturated, and A
The addition of l only increases the alloy cost, and further increases the Al ₂ O ₃ concentration in the slag due to the secondary decarburization refining, making it difficult to produce molten steel with an extremely low Al concentration.

Here, the composition of the slag in the desulfurization treatment using the converter type container has a basicity of 1.3 to 1.8, A
The l ₂ O ₃ may be 10 wt% or less. This allows
The desulfurization ability of the slag can be enhanced, and the generation of free CaO can be suppressed to prevent the slag from expanding, thereby making it possible to use the slag as a resource. When the basicity of the slag is less than 1.3, the desulfurization ability of the slag decreases, and the desulfurization reaction deteriorates. On the other hand, when the basicity of the slag exceeds 1.8, free CaO is generated as the basicity increases, the slag expands greatly, and poor slagging is likely to occur. Further, the concentration of Al ₂ O ₃ in the slag is 10
If the content is more than 10% by weight, the desulfurization ability of the slag decreases, and the desulfurization reaction deteriorates. The lower limit of the concentration of Al ₂ O ₃ in the slag is preferably 3% by weight in order to promote slag slag and improve desulfurization reaction.

Furthermore, the Cr ₂ O ₃ concentration in the slag exit the reduction treatment may be be 5 wt% or less. As a result, expensive Cr contained in the slag can be reduced and recovered in the crude molten steel, and the cost of ferroalloys can be reduced. When the concentration of Cr ₂ O ₃ in the slag exceeds 5% by weight, Al added in the desulfurization treatment is consumed for the reduction of Cr ₂ O ₃ , and the oxygen potential in the slag is insufficiently reduced, and the desulfurization reaction is reduced. I do.

It is preferable that the temperature of the molten steel at the time of the reduction treatment and the desulfurization treatment is 1550 ° C. or more and less than 1700 ° C. Thereby, slagification of slag is promoted, desulfurization reaction is promoted, and low-melting stainless steel with low S can be easily produced. If the temperature of the molten steel at the time of the reduction treatment and the desulfurization treatment is lower than 1550 ° C., it takes a long time to make the slag slag, and the desulfurization reaction is disadvantageous. On the other hand, when the temperature of the molten steel is 1700 ° C. or higher, the temperature becomes too high, the erosion of the refractory in the refining furnace increases, and the refractory cost increases.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described with reference to the accompanying drawings to provide an understanding of the present invention. FIG. 1 is an explanatory diagram showing a flow of processing by a method for producing molten stainless steel according to one embodiment of the present invention. As shown in FIG. 1, the decarburizing and refining process of molten stainless steel according to the method for producing molten stainless steel according to one embodiment of the present invention is performed using an upper-bottom blow-roller, which is an example of a converter-type vessel used for primary decarburization refining. A furnace 10 and a vacuum ladle refining furnace 30 which is an example of a secondary refining furnace used for secondary decarburization refining are used. The top-bottom blowing converter 10 includes a furnace body 11 for containing molten iron and a ferrochrome alloy, a blowing acid lance 13 disposed above a furnace port 12 so as to be movable up and down to blow oxygen to the molten iron and ferrochrome alloy, and a bottom part of the furnace body 11. And a bottom blow nozzle 16 for injecting an argon gas, which is an example of an inert gas, to agitate the slag 14a and the crude molten steel (also referred to as crude stainless steel) 15a, and a steel tapping port 17 at the upper side of the furnace body 11. I have.
Further, a hopper 18 for storing CaO, ferrosilicon (Fe—Si), Al, etc., which are examples of auxiliary materials, above the furnace port 12 and a chute 19 disposed below the hopper 18,
A ladle 2 having a porous plug 20 for receiving a slag 14b subjected to desulfurization treatment in a furnace and receiving a smelted stainless steel melt 15b and blowing an inert gas into a bottom thereof.
1 is provided.

[0010] The vacuum ladle refining furnace 30 is an upper-bottom blow converter 10
Ladle 21 receiving stainless steel molten steel 15b smelted in
And a pressure reducing hood 36 covering the ladle 21, a blowing acid lance 31 penetrating through the pressure reducing hood 36 and communicating with the inside, a pressure reducing duct 32 communicating with an ejector (not shown), and a shutoff valve in the middle. CaO with 34
33 connected to a storage hopper 35 of steel or alloy iron
have.

Next, the method for smelting stainless steel according to the present invention will be described using an upper-bottom blow converter 10 and a vacuum ladle refining furnace 30. The molten iron and the ferrochrome alloy are charged into the furnace from the furnace port 12 of the top and bottom blown converter 10, and the blowing acid lance (top blowing lance) 13 is lowered from the furnace port 12 into the furnace.
Oxygen of 00 to 25000 Nm ³ / (hour) is sprayed, and at the same time, CaO is added from the hopper 18 and primary decarburization refining is performed by blowing acid from the blowing acid lance 13. In this primary decarburization refining, the ferrochrome alloy is rapidly melted by the increase in heat by the blowing acid, and the so-called decarburization reaction in which oxygen and carbon react is promoted, and the carbon concentration becomes 0.3 to 0.1.
A stainless steel molten steel 15a having a temperature of 1550 ° C. or more and less than 1700 ° C. is melted at 8% by weight. During this primary decarburization refining, added CaO, hot metal and silicon (Si) and Al contained in the ferrochrome alloy are oxidized to form slag 14a. The slag 14a is formed of Cr ₂ O ₃ in which Cr in the ferrochrome alloy is oxidized during the blowing acid of the primary decarburization refining.
, A Fe-Si alloy, which is an example of an alloy containing silicon, is added from a hopper 18, and argon gas is blown into the stainless steel molten steel 15a from the bottom blow nozzle 16 to remove the slag 14a and the stainless steel molten steel 15a. Stirring is performed to reduce Cr ₂ O ₃ . This reduction process
Performed until the Cr ₂ O ₃ concentration in the slag 14a becomes 5 wt% or less. Then, after the reduction process is completed, while blowing the argon gas from the bottom blowing nozzle 16 is continued,
A metal Al, which is an example of Al, is shot from the hopper 18 into the chute 1
9 to add the Al concentration to be 0.05 to 0.2% by weight, and agitate the slag 14a and the molten stainless steel 15a to perform desulfurization treatment. This desulfurization treatment immediately before, and analyzed by sampling in advance slug 14a, the slag basicity 14a (CaO / SiO ₂₎ is 1.3 to 1.8, Al ₂
The amount of CaO and the amount of metal Al added into the furnace via the chute 19 are adjusted so that O ₃ becomes 10% by weight or less.

In the reduction treatment using the upper and lower blown converters 10, the temperature of the molten stainless steel 15a and the slag 14a is reduced to 155.
0 ° C. or more and less than 1700 ° C., argon gas was blown in from the bottom blowing nozzle 16 and stirred, and moreover, Fe—Si
Since the alloy is added, slag of the slag 14a becomes good, and the reduction reaction of Cr ₂ O ₃ in the slag 14a is promoted. The reduced Cr is recovered in the molten stainless steel 15a, and the concentration of Cr ₂ O ₃ remaining in the slag 14a can be reduced to 5% by weight (weight% in terms of Cr) or less. In the desulfurization treatment performed after the reduction treatment, metal Al is added to the crude molten steel 15a and the concentration thereof is set to 0.05 to
Since it is 0.2% by weight, the oxygen potential in the slag 14a is sufficiently reduced, and the temperature is 1550 ° C.
Since the temperature is set to less than 0 ° C. and the gas is stirred by blowing argon gas from the bottom blowing nozzle 16, the desulfurization reaction is promoted and the ultimate S concentration can be reduced. Furthermore, slag 14
Since the concentration of Cr ₂ O ₃ in a have a 5 wt% or less, the added metallic Al to reduce the depletion due to oxidation reaction of the Cr ₂ O _3, moreover, Al ₂ O in the slag 14a
₃ , the oxygen potential in the slag 14a can be efficiently reduced, and the desulfurization reaction can be stably promoted. The stainless steel crude molten steel 15b that has been subjected to the reduction treatment and the desulfurization treatment has an S concentration of 0.00 by the desulfurization treatment.
05 to 0.001% by weight, the furnace body 11
By the tilting of the tapping, the steel is tapped from the tapping port 17 to the ladle 21, and is moved to a treatment place by the vacuum ladle refining furnace 30 by a transport means (not shown). Further, the slag 14b is formed of the residual Cr.
The concentration of ₂ O ₃ is 5% by weight or less, and the content of CaO / SiO ₂ is 1.
_{3 to} 1.8, Al ₂ O ₃ becomes 3 to 10% by weight, and as a high quality slag without expansion, a furnace port 12
After cooling and crushing, it is effectively used as resources such as roadbed materials and civil engineering landfill materials.

Next, in the vacuum ladle refining furnace 30, a vacuum hood 36 is placed over the ladle 21 containing the stainless steel molten steel 15b, the shutoff valve 34 of the chute 33 is opened, and the storage hopper 35 is opened. , An auxiliary material such as CaO is charged into the ladle 21, the blowing acid lance 31 is lowered to start blowing acid, and at the same time, the ejector is operated to move the ladle 21 from the vacuum duct 32.
Was heated to a reduced pressure of 100 to 0.2 torr, and secondary decarburization refining was performed until the carbon concentration became 0.01 to 0.05% by weight. After the end of the secondary decarburization refining, the stainless steel molten steel 15b was deoxidized without adding Al.

[0014] By blowing oxygen from the blowing acid lance 31, when metal Al is added in the desulfurization treatment, the Al added to the molten steel 15b is reliably oxidized and formed by secondary decarburization refining. Al ₂ O on the slag 14c
_{Since it} is absorbed as ₃ , and Al is not used in the deoxidation treatment, addition of Al to the stainless steel molten steel 15c can be prevented. Further, Al remaining in the stainless steel molten steel 15b
Is oxidized during the secondary decarburization refining using a vacuum ladle refining furnace 30 Al ₂ O _3, and the Part Al ₂ O ₃ generation amount are the very slight, Al ₂ O in the composition of the slag 14c
_{3 The} concentration does not change as much as the difference.
There is an advantage that Al ₂ O _{3 in} 4c is not reduced, and by synergistic action, extremely low Al stainless steel molten steel 15c having an Al concentration of 0.001 to 0.002% by weight could be produced. Then, the molten stainless steel 15c having a carbon concentration of 0.01 to 0.05% by weight, an extremely low Al content and a low S content is formed into a slab using continuous casting or the like, and cut into a predetermined size.
Processing such as rolling is performed to produce a steel plate, a mold steel, and the like.

[0015]

Next, an embodiment of a method for producing molten stainless steel according to the present invention will be described. Hot metal and Fe-Cr are charged into an upper and lower blown converter (converter type vessel), a blowing acid lance is lowered into the furnace, and oxygen is blown to carry out a heat treatment to raise the carbon concentration to 0.3 to 0. Primary decarburization refining was carried out until the content reached 8% by weight, and a crude stainless steel was melted. Furthermore, when smelting stainless steel crude molten steel, the Al concentration and slag composition (CaO / SiO ₂ , Al ₂ O ₃ concentration) at the time of desulfurization treatment, the Cr ₂ O ₃ concentration at the end of reduction treatment, The reduction treatment and the desulfurization treatment were performed while changing the temperature. Then, S distribution during desulfurization ((% S) / [% S]), controllability of extremely low Al reduction during secondary refining, converter refractory erosion, resource utilization of slag after desulfurization, and comprehensive evaluation were investigated. . Table 1 shows the results.
Shown in In Examples 1 and 2, the Al concentration during converter desulfurization, the slag composition during converter desulfurization, and the Cr ₂ O ₃
Both the concentration and the temperature at the time of the reductive desulfurization treatment satisfy the range of the present invention.
0, 670, good controllability of ultra-low Al reduction during secondary refining, reduction of converter refractory erosion, recycling of slag after desulfurization, good overall evaluation (○) Obtained. Example 3 shows the case where Al during converter desulfurization was used.
In Example 4, the Al concentration at the time of converter desulfurization was set to the upper limit of 0.20% by weight, and the S distribution at the time of desulfurization was 420 and 570, respectively.
And the controllability of extremely low Al during secondary refining was good, the refractory erosion of the converter could be reduced, the slag could be recycled after desulfurization, and a good overall evaluation (○) was obtained. . In Example 5, the slag composition (CaO /
SiO ₂ ) is the lower limit of 1.3, and Example 6 is the case where the slag composition (CaO / SiO ₂ ) at the converter desulfurization is 1.8 at the upper limit. 3
20 and 750, good controllability of ultra-low Al reduction during secondary refining, reduction of converter refractory erosion, recycling of slag after desulfurization, good overall evaluation (○)
The result was obtained. Example 7 10 wt% BOF desulfurization during slag composition of (Al ₂ O ₃ concentration) which is the upper limit, the embodiment 8, BOF desulfurization during slag composition of (Al ₂ O ₃ concentration) 4.
In this case, the S distribution during desulfurization was 4% each.
30, 410, good controllability of ultra-low Al reduction during secondary refining, reduction of converter refractory erosion, resource recycling of slag after desulfurization, good overall evaluation (○)
The result was obtained. Example 9 is the case where the temperature during the reductive desulfurization treatment is 1550 ° C., and Example 10 is the case where the temperature during the reductive desulfurization treatment is 1699 ° C., and the S distribution during the desulfurization can be 330 and 640, respectively. The controllability of ultra-low Al reduction in the secondary refining was good, the refractory erosion of the converter could be reduced, the slag could be recycled after desulfurization, and good (、) results were obtained as a comprehensive evaluation.

[0016]

[Table 1]

On the other hand, in Comparative Example 1, A
This is the case where the l concentration is as low as 0.02% by weight, the S distribution during desulfurization is extremely bad at 30, and the load of secondary decarburization refining is increased in order to reduce the S, and the overall evaluation is bad (x) The result was. In Comparative Example 2, the Al concentration during converter desulfurization was 0.1.
32% by weight, which is very low during secondary refining.
poor controllability of Al in stainless steel molten steel
The concentration could not be reduced to an extremely low Al concentration, and the result was poor (x) as the overall evaluation.

The embodiment of the present invention has been described above.
The present invention is not limited to the above-described embodiment, and all changes in conditions that do not depart from the gist are within the scope of the present invention. For example, Al added during desulfurization treatment is metal Al
In addition, Al alloys such as Fe-Al and aluminum dross can be used. Furthermore, DH, RH and V which are generally used as a decompression refining apparatus are used as secondary decarburization refining furnaces.
OD or the like can be used.

[0019]

According to the present invention, the molten iron and the ferrochrome alloy are charged into a converter type vessel, and the molten iron and the ferrochrome alloy are acid-blended using an upper blowing lance. After melting the hot metal and the ferrochrome alloy and performing the primary decarburization refining to produce the crude molten steel, the crude molten steel is tapped into a ladle, and then the crude molten steel is subjected to secondary decarburization refining using a secondary refining furnace. In the smelting method of stainless steel molten steel for performing deoxidation treatment, an alloy containing silicon is added to slag generated after primary decarburization refining by blowing acid to reduce the slag by reducing the slag. Heat is added in a secondary decarburization refining using a secondary refining furnace, in which Al is added so that the concentration becomes 0.05 to 0.2% by weight, desulfurization treatment is performed, and then steel is output to a ladle. Since the treatment is performed without adding Al during the deoxidation treatment, Reducing the Al concentration in the stainless molten steel, to promote desulfurization, very low Al, to industrially stably can produce ingot of low S stainless molten steel at low cost.
Moreover, it can be used as resources such as slag roadbed materials and civil engineering landfill materials generated by decarburization refining.

In particular, in the smelting method for molten stainless steel according to the _second aspect, the slag composition at the time of desulfurization treatment using a converter type vessel has a basicity of 1.3 to 1.8, and Al ₂ O _3. To 10% by weight or less, it is possible to stably produce extremely low Al and low S stainless steel molten steel, stably suppress expansion caused by free CaO, and promote slag resource recycling. it can.

[0021] In the melting method of claim 3 stainless molten steel according, Cr ₂ O ₃ in the slag finishing the reduction treatment
Since the concentration is 5% by weight or less, expensive Cr can be reduced and recovered in the crude molten steel, and the cost of ferroalloys can be reduced.

In the method for producing molten stainless steel according to claim 4, the temperature of the molten steel at the time of the reduction treatment and the desulfurization treatment is set to one.
Since the temperature is set to 550 ° C. or higher and lower than 1700 ° C., slag formation is promoted, a desulfurization reaction is promoted, and low-melting stainless steel with a low S can be easily produced, and slag formation is promoted. Slag can be stably utilized as a resource by suppressing free CaO.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a flow of processing by a method for producing molten stainless steel according to an embodiment of the present invention.

[Explanation of symbols]

10: top and bottom blown converter (converter type vessel), 11: furnace body, 1
2: furnace port, 13: blowing acid lance, 14a: slag, 14
b: slag, 14c: slag, 15a: stainless steel molten steel, 15b: stainless steel molten steel, 15c: stainless steel molten steel, 16: bottom blow nozzle, 17: tapping outlet, 18: hopper, 19: chute, 20: porous plug, 21: Ladle, 30: Vacuum ladle refining furnace (secondary refining furnace), 31: Blow acid lance, 32: Vacuum duct, 33: Chute, 34: Shutoff valve, 35: Storage hopper, 36: Vacuum hood

Claims (4)

[Claims] Claims 1. A converter type vessel is charged with hot metal and a ferrochrome alloy, and the hot metal and the ferrochrome alloy are subjected to blowing acid using an upper blowing lance to perform melting and primary decarburization refining of the hot metal and the ferrochrome alloy. After smelting the crude molten steel, the crude molten steel is tapped in a ladle, and then the secondary molten metal is subjected to secondary decarburization refining and deoxidation using a secondary smelting furnace. After reducing the slag by adding an alloy containing silicon to the slag generated after performing the primary decarburization refining with the blowing acid, the A in the crude molten steel is reduced.
Secondary decarburization refining using the secondary refining furnace is performed after Al is added so that the l concentration becomes 0.05 to 0.2% by weight and desulfurization treatment is performed, and then steel is output to the ladle. A method for producing molten stainless steel, wherein Al is not added at the time of heat-up and deoxidation treatment. 2. The method for producing molten stainless steel according to claim 1.
In the method, during the desulfurization treatment using the converter type container
The slag composition has a basicity of 1.3 to 1.8, Al _Two O_Three 
Stainless steel, characterized in that the
Steel melting method. 3. The method for producing molten stainless steel according to claim 1, wherein the concentration of Cr ₂ O ₃ in the slag after the reduction treatment is reduced to 5% by weight or less. Melting method. 4. The method for producing molten stainless steel according to claim 1, wherein the temperature of the molten steel during the reduction treatment and the desulfurization treatment is 1550 ° C. or more and 170 ° C.
A method for producing molten stainless steel, which is performed at a temperature lower than 0 ° C.